Tropical Southeast Asian countries said yesterday they were bracing for the mosquito-borne Zika virus, with Malaysia saying it could "spread quickly" if introduced, but Thailand appeared to be bucking the trend with just a handful of cases a year.

Zika, linked to severe birth defects including babies born with abnormally small heads, is wreaking havoc in Brazil where the government has deployed more than 200,000 troops to eradicate mosquitoes.

The World Health Organization (WHO) said on Thursday that the virus was "spreading explosively" and could infect as many as four million people in the Americas.

No treatment or vaccine is available.

Thailand detected its first Zika case in 2012 and has recorded an average of five cases a year, according to the Ministry of Public Health.

Thailand has confirmed one case of the virus so far this year. Earlier this month, Taiwan reported one case of Zika infection in a man from northern Thailand.

The WHO's Western Pacific Region Office in Manila said as long as Aedes mosquitoes circulated in the region "it can be anticipated that the virus will emerge".

Malaysia's Health Ministry said Zika had not yet been detected. "If it is introduced by an infected Malaysian or by a visitor to Malaysia, it could spread quickly," said Lokman Hakim Sulaiman, Malaysia's Health Ministry deputy director-general.

Neighbouring Singapore has not detected any Zika infections but the government said there was a high risk of transmission if cases were imported to Singapore, a regional travel hub, reports Reuters.

In the Western Pacific, Zika was first reported in Micronesia in 2007. It was reported in French Polynesia in October 2013, and since then, a number of Pacific Island countries have reported cases, including New Caledonia, Cook Islands, Solomon Islands, Vanuatu, Fiji and Samoa.

In Australia, the foreign ministry's travel advice website said there had been no reported cases of Zika.

The New Zealand Herald reported yesterday that one local man had been admitted to hospital with symptoms linked to the Zika virus. The Ministry of Health said it had received nine Zika notifications this year, the newspaper reported.

All of the travellers had been in the Pacific Islands and eight of them had recovered.

Zika virus is spreading rapidly through the Americas leaving researchers scrambling to understand the very basics, including how to prevent, treat and diagnose the emerging mosquito-borne threat.

No matter how fast the world acts, it will be years before a vaccine is widely available, said Anthony Fauci, head of the National Institute for Allergy and Infectious Diseases (NIAID).

"This is a brand-new virus so we, prior to this time, have really not spent anything on Zika," Fauci said.

Nor is there any vaccine on the market yet against dengue virus, which comes from the same family of flaviviruses.

The US government is appealing for all kinds of research on Zika and will draw from a pool of $97 million to fund studies on everything from how it spreads to how it causes disease to how to control the mosquitoes that spread it, reports AFP.

"NIAID researchers are working on vaccine candidates to prevent Zika virus infection," he added.

"It is to our advantage that we already have existing vaccine platforms to use as a sort of jumping off point," he said, mentioning two avenues of study based on prior research into West Nile and dengue vaccines.

"While these approaches are promising, it is important to understand that we will not have a widely available, safe and effective Zika vaccine this year and probably not even in the next few years."

Zika was first identified in 1947, causing its first human case in 1952 in Uganda. However, most cases were mild, resulting in rash, fever, and red eyes in a small fraction of cases.